1. Field of the Invention
The present invention relates to area sensors for use in digital cameras, X-ray photographing apparatuses, etc., relates to image input apparatuses, such as digital cameras and X-ray photographing apparatuses having area sensors, and relates to methods for driving the area sensors. More particularly, in an area sensor in which pixels having photoelectric conversion elements and switching elements are arrayed two-dimensionally, the present invention relates to an area sensor in which a driving signal for driving each pixel is applied from opposing two sides, relates to an image input apparatus having the area sensor, and relates to a method of driving the area sensor.
2. Description of the Related Art
An example of an area sensor in which pixels having photoelectric conversion elements and switching elements are arrayed two-dimensionally is described with reference to FIGS. 1, 2A, 2B, 2C, and 3.
FIG. 1 is a schematic circuit diagram of an example of an area sensor having photoelectric conversion elements and switching elements in pixels. FIG. 2A is an equivalent circuit diagram of a gate line of an area sensor. FIGS. 2B and 2C are diagrams illustrating the status of a driving waveform applied to the gate line from a gate driver. Also, FIGS. 2B and 2C are conceptual diagrams illustrating the shape of a driving waveform (gate pulse) at point A and point B, respectively, in FIGS. 1 and 2A. FIG. 3 is a timing chart illustrating an example of the driving of the gate driver in the area sensor.
Each pixel S of the area sensor shown in FIG. 1 is formed of a photoelectric conversion element (herein, a photodiode) PD and a thin-film transistor (TFT) Tr. The cathode electrode of the photodiode PD is connected to a bias line Vs, and a bias voltage is applied by a power supply 3. The source electrodes of the thin-film transistors Tr are connected to the data lines Sig1 to SigN for each pixel column, and the gate electrodes of the thin-film transistors Tr are connected to the gate lines Vg1 to VgN for each pixel row. In this example, wiring necessary for driving the area sensor is the bias line Vs, the data lines Sig1 to SigN, and the gate lines Vg1 to VgN. Furthermore, the anode electrodes of the photodiodes PD and the drain electrodes of the thin-film transistors Tr are connected to each other by pixels. In this example, the data lines Sig1 to SigN are arranged in the vertical direction in FIG. 1, and the gate lines Vg1 to VgN are arranged in the horizontal direction. In addition, each of the data lines Sig1 to SigN is connected to a reading apparatus 1. Generally, the reading apparatus 1 comprises an amplifier 1a to which each of the data lines Sig1 to SigN is connected, and an analog multiplexer 1b to which a signal from each amplifier 1a is input. On the other hand, each of the gate lines Vg1 to VgN is connected to a gate driver 2. Generally, the gate driver 2 comprises a shift register.
Image information which is photoelectrically converted by each pixel is transferred to the reading apparatus 1 through the data lines Sig1 to SigN, and is output as a serial signal for each gate line.
In the area sensor, the gate driver and the reading apparatus are connected to each other only at one side of a rectangular area which is an image receiving area in which pixels are arrayed. However, the area sensor having such a connection relationship has a problem in that, in a case where a disconnection occurs in the gate line or the data line, the driving of pixels in a portion after the disconnection and the transfer of a signal from the pixels cannot be performed.
Also, even when a disconnection has not occurred, the gate line of the area sensor includes a resistor. In particular, when the image receiving area is enlarged, or when the pixels are arranged in finer lines and the gate line width is reduced, cases occur in which the resistance cannot be substantially ignored. The gate line of the area sensor can be expressed by a resistor Rvg of the gate line itself and a parasitic capacitor Cvg of the gate line, as in FIG. 2A, from a point of view of an equivalent circuit. Therefore, in the gate pulse (see FIG. 2B) which is applied to “A” of the gate line VgN in FIG. 1, a delay occurs in the signal at “B” of the gate line VgN due to the resistor Rvg and the parasitic capacitor Cvg, the waveform is deformed as shown in FIG. 2C, and a wavelength width in a portion exceeding a threshold voltage Vth is varied. Generally, in order to turn on a TFT and to transfer electric charge, it is necessary for the time period during which a voltage exceeding the threshold voltage Vth to be a time period Tb or more. When such a delay of a waveform as that described herein occurs, since the time period in which this threshold voltage Vth is exceeded becomes short, the time period Ta of the original pulse width must be set to be long so as to secure the time period Tb required for transferring electric charge.
As shown in the driving timing chart of FIG. 3, in order to drive the entirety of the area sensor, due to this delay, a time period becomes necessary which is extended by an amount corresponding to (Ta−Tb)×N (N is the number of lines). There are cases in which this becomes an obstacle to driving the area sensor at an even higher speed.
As described above, the area sensor could be further improved in view of the fact that the driving of pixels in a portion after a disconnection and the transfer of a signal from the pixels cannot be performed and in order to perform high-speed driving.